hmac --- Keyed-Hashing for Message Authentication

Source code: Lib/hmac.py

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This module implements the HMAC algorithm as described by RFC 2104. The interface allows to use any hash function with a fixed digest size. In particular, extendable output functions such as SHAKE-128 or SHAKE-256 cannot be used with HMAC.

hmac.new(key, msg=None, digestmod)

Return a new hmac object. key is a bytes or bytearray object giving the secret key. If msg is present, the method call update(msg) is made. digestmod is the digest name, digest constructor or module for the HMAC object to use. It may be any name suitable to hashlib.new(). Despite its argument position, it is required.

在 3.4 版的變更: Parameter key can be a bytes or bytearray object. Parameter msg can be of any type supported by hashlib. Parameter digestmod can be the name of a hash algorithm.

在 3.8 版的變更: The digestmod argument is now required. Pass it as a keyword argument to avoid awkwardness when you do not have an initial msg.

hmac.digest(key, msg, digest)

Return digest of msg for given secret key and digest. The function is equivalent to HMAC(key, msg, digest).digest(), but uses an optimized C or inline implementation, which is faster for messages that fit into memory. The parameters key, msg, and digest have the same meaning as in new().

CPython implementation detail, the optimized C implementation is only used when digest is a string and name of a digest algorithm, which is supported by OpenSSL.

在 3.7 版被加入.

An HMAC object has the following methods:

HMAC.update(msg)

Update the hmac object with msg. Repeated calls are equivalent to a single call with the concatenation of all the arguments: m.update(a); m.update(b) is equivalent to m.update(a + b).

在 3.4 版的變更: Parameter msg can be of any type supported by hashlib.

HMAC.digest()

Return the digest of the bytes passed to the update() method so far. This bytes object will be the same length as the digest_size of the digest given to the constructor. It may contain non-ASCII bytes, including NUL bytes.

警告

When comparing the output of digest() to an externally supplied digest during a verification routine, it is recommended to use the compare_digest() function instead of the == operator to reduce the vulnerability to timing attacks.

HMAC.hexdigest()

Like digest() except the digest is returned as a string twice the length containing only hexadecimal digits. This may be used to exchange the value safely in email or other non-binary environments.

警告

When comparing the output of hexdigest() to an externally supplied digest during a verification routine, it is recommended to use the compare_digest() function instead of the == operator to reduce the vulnerability to timing attacks.

HMAC.copy()

Return a copy ("clone") of the hmac object. This can be used to efficiently compute the digests of strings that share a common initial substring.

A hash object has the following attributes:

HMAC.digest_size

The size of the resulting HMAC digest in bytes.

HMAC.block_size

The internal block size of the hash algorithm in bytes.

在 3.4 版被加入.

HMAC.name

The canonical name of this HMAC, always lowercase, e.g. hmac-md5.

在 3.4 版被加入.

在 3.10 版的變更: Removed the undocumented attributes HMAC.digest_cons, HMAC.inner, and HMAC.outer.

This module also provides the following helper function:

hmac.compare_digest(a, b)

Return a == b. This function uses an approach designed to prevent timing analysis by avoiding content-based short circuiting behaviour, making it appropriate for cryptography. a and b must both be of the same type: either str (ASCII only, as e.g. returned by HMAC.hexdigest()), or a bytes-like object.

備註

If a and b are of different lengths, or if an error occurs, a timing attack could theoretically reveal information about the types and lengths of a and b—but not their values.

在 3.3 版被加入.

在 3.10 版的變更: The function uses OpenSSL's CRYPTO_memcmp() internally when available.

也參考

Module hashlib

The Python module providing secure hash functions.